The present invention relates to a f our-wheel drive torque transfer mechanism for selectively distributing torque between the front and the rear wheels of an automobile.
Conventional drive systems for running automobiles include front wheel drive systems and rear wheel drive systems. A front wheel drive system is generally more stable than a rear wheel drive system. However, in making a turn, a front wheel drive system loses cornering power to the extent that a large steering angle must be maintained.
While a rear wheel drive system gives more satisfactory turning performance, the use of the rear wheel drive system may lead to over-turning when the torque is too high.
Thus, ideally, the driving torque should be distributed equally to the front and the rear wheels. Toward this end, a four-wheel drive vehicle has been proposed in which the torque of the engine is distributed to the front and the rear wheels by a central differential gear. In the power transmission of such a four-wheel drive vehicle, the engine torque is first transmitted to the central differential gear, which consists of a gear unit composed of pinions having side gears on both sides thereof, or a planetary gear unit. The torque transmitted to the central differential gear is distributed forwardly and rearwardly. The forwardly distributed torque is transmitted to a front-wheel differential gear, where it is distributed again by right and left side gears to the right and the left front wheels. The rearwardly distributed torque is transmitted through a rear-wheel-drive propeller shaft to a rear-wheel differential gear, where it is further distributed by right and left side gears to the right and the left rear wheels.
The central differential gear prevents the occurrence of the so-called tight corner braking, a phenomenon in which a difference in rotation between the front and the rear wheels causes a braking action when a vehicle makes a sharp turn at low speed on a road surface having a low coefficient of friction. Further, the central differential gear helps to increase the torque-transmission limit value during turning. In addition, this gear maximizes the traction force when the front and the rear wheels are driven by the same torque.
However, if any one of the four driving wheels of the four-wheel drive vehicle is brought into a no-load state as a result of slipping, for example, on a snowy or frozen road surface, the differential operation of the central differential gear will cause the drive energy to escape through that slipping wheel, thereby making the transmission of torque to the wheels with traction impossible.
In view of the foregoing problem, a differential limiting mechanism for stopping the differential operation of the central differential gear is conventionally provided. The differential limiting mechanism is composed of a wet-type multi-disc friction clutch and a hydraulic servo. Under normal operating conditions, the multi-disc friction clutch is released to maintain the central differential gear in the differential operating mode, so that the torque is distributed to the front and the rear wheels. If any one of the wheels slips or threatens to slip on a snowy or frozen road surface or the like, the multi-disc friction clutch is engaged to bring the central differential gear into a direct drive mode.
The slipping limit characteristic of a driving wheel is determined by the vertical load on the surface of the tire which is in contact with the ground, and this load varies depending mainly upon the weight distribution of the vehicle. Accordingly, when the central differential gear is brought into the direct drive position, not only does a loss occur in torque transmission, but the parts of the multi-disc friction clutch, the gears, etc. are subjected to wear. Further, when the torque is gradually increased in the mode where the multi-disc friction clutch is released and the central differential gear is in the differential operating mode, either the front or the rear wheels will slip, making a further increase in torque impossible. In other words, the torquetransmission limit value is determined by the smaller of the two values of torque distributed to the front and the rear wheels. If, for example one, of the front wheels slips, the torque for the rear wheels will be reduced instantaneously, thereby making it impossible to transmit torque satisfactorily to the road surface.
In view of the foregoing, a four-wheel drive torque controller has been proposed in which a differential limiting clutch is arranged in parallel with the central differential gear. The engagement, i.e. the amount of slip, of this differential limiting clutch is continuously controlled by a controller, thereby increasing the torque-transmission limit valve, gradability and traction force of the vehicle and improving the stability of the vehicle when running on a snowy or frozen road or the like (see Japanese Patent Laid-Open No. 63-176728).
Further, a four-wheel drive torque controller has been proposed in which the torque ratio is continuously controlled to improve the turning and maneuvering characteristics and running ability of the vehicle (see Japanese Patent Laid-Open No. 63-34230).
The use of such a four-wheel drive torque controller in a four-wheel drive vehicle provides the advantage that any change in the steering angle during turning is detected by a steering angle sensor, and, on the basis of the value of the detected steering angle, the angular velocity (the yaw rate) of this steering angle is calculated. In accordance with this calculated yaw rate the controller controls the slippage of the differential limiting clutch. Thus, it is possible to distribute torque in a fixed ratio as needed during turning, thereby improving the turning and maneuvering characteristics of the vehicle.
However, the turning and maneuvering characteristics of a four-wheel drive vehicle when it actually makes a turn are influenced not only by the yaw rate of the steering angle, but also by the vehicle velocity, vehicle lateral acceleration, etc. Unless an appropriate torque distribution is effected in accordance with all these factors, the vehicle will fall into an understeering or an oversteering condition.